The present invention generally relates to solder preforms of the type used to attach leaded circuit components to circuit boards. More particularly, this invention relates to a two-part solder preform formed by two different solder compositions, a lower-temperature solder alloy and a higher-temperature solder alloy, so that the preform can be flowed to form a solder joint at a lower temperature than the melting temperature of the higher-temperature solder alloy.
Circuit components are at times electrically and physically connected to circuit boards with wire leads. Such leaded components are mounted to a circuit board by inserting their leads in plated through-holes provided in the circuit board, and then soldering the leads to metallization (e.g., metal runners) on the opposite side of the board, forming side-to-side connections. The solder may be applied by wave soldering or provided in the form of a paste applied to the leads, and then flowed to form solder joints. If wave soldering or the use of a solder paste is not practical or possible, a less widely used alternative is to use solder preforms that are assembled individually on the leads of a leaded component. Preforms are usually formed as rings, but may be formed from a flat sheet material that is stamped to yield various complex shapes that can be assembled to the leads of certain leaded components.
A current trend in the electronic industry is to reduce the amount of lead used in the processing and assembly of electronic devices. Many of the more suitable candidates for lead-free solder alloys have melting or liquidus temperatures significantly higher than the often-used Sn63xe2x80x94Pb37 eutectic alloy (melting temperature of 183xc2x0 C.). Because many electronic components and processes are designed and validated for reflow temperatures developed for tin-lead alloys, there is a concern that the temperatures required to reflow lead-free solder alloys could potentially damage the component being soldered as well as surrounding components on the same circuit board. While components and processes that may be sensitive to higher reflow temperatures can be developed and validated for the higher reflow temperatures of the lead-free solder alloys, such an undertaking is very costly.
U.S. Pat. No. 5,803,340 to Yeh et al. discloses a solder paste composition that contains fine powders of two or more different solder alloys, one of which has a lower melting temperature than the other. The solder paste is characterized by the ability to flow at the melting temperature of the lower-melting alloy, and then upon subsequent reheating to the melting temperature of the higher-melting alloy, remelt both alloys to yield a solder joint having a homogenous composition. Such two-component solder pastes have found use for flip chips and other surface-mount technology (SMT) devices attached to circuit boards with solder bumps that are formed on the devices and then reflowed to solder the devices to the board. Solder pastes in accordance with Yeh et al. provide a lower bumping temperature and a higher reflow temperature to yield a solder joint capable of withstanding high temperatures as a result of having a melting temperature between the bumping and reflow temperatures of the original paste. However, such an approach has not been applied to solder preforms used for leaded circuit components. One reason is that leaded components are mounted to circuit boards after SMT devices have been mounted and reflowed. SMT devices are vulnerable to downstream processes that can raise the temperature of their fine-pitch solder connections above the solder melting temperature, causing the solder connections to reflow and create shorts and open circuits. If a preform containing lower and higher-melting solder alloys were used, one would expect that the process of soldering a leaded component to a circuit board with the preform would induce unacceptably high thermal stresses in adjacent SMT devices as well as the substrate, device packages and connections. In addition, there is a concern that adjacent SMT solder connections, and particularly those of fine-pitch flip chips and other fine-pitch SMT devices, would reflow unless both the lower and higher-melting solder alloys of the preform had melting temperatures that are sufficiently below the melting temperature of the SMT solder connections. As previously discussed, another drawback would be that the leaded components and their processing would require costly revalidation at the reflow temperature of the higher-melting solder alloy.
The present invention provides a two-part solder preform for use in attaching a leaded circuit component to a circuit board. The solder preform is formed by two different solid components, a first of which is formed of a lower-temperature solder alloy and the second of a higher-temperature solder alloy. According to the invention, when present in a sufficient amount, intimate contact between the first and second solid components enables the first solid component to dissolve the second solid component when the preform is heated above the melting temperature of the lower-temperature solder alloy but below the melting temperature of the higher-temperature solder alloy.
Solder preforms in accordance with this invention may be provided in several different forms. For use with leaded circuit components, the preform preferably has an annular shape, though various other shapes and forms could be suitable. In one form, the first and second solid components are formed separately, and then stacked to form the preform. In another form, the preform is a unitary member in which the first and second solid components are mechanically joined, such as by lamination. In a third form, the first and second solid components are powders that are mixed and compacted together to form the preform. In each case, the two-part solid solder preform of this invention is formed by mechanically combining the first and second solid components. After assembling the preform to a lead of the circuit component, the preform is heated to a temperature above the melting or liquidus temperature of the lower-temperature alloy of the first solid component, but below the liquidus temperature of the higher-temperature alloy of the second solid component. According to the invention, the first solid component constitutes a sufficient amount of the preform so that when the first solid component melts, the lower-temperature alloy dissolves substantially all of the higher-temperature alloy. Upon cooling, the two-part solid solder preform yields a solder joint having a substantially homogenous composition, and bonds the lead to the circuit board. Preferred solder alloys for the first and second solid components are lead-free, with the alloy for the second solid component and the composition of the final solder joint having melting temperatures well above that of the conventional Sn63xe2x80x94Pb37 eutectic alloy (melting temperature of 183xc2x0 C.).
In view of the above, it can be seen that a significant advantage of the present invention is that it provides a method for soldering a leaded component to a circuit board using a soldering temperature below the melting temperature of the higher-melting component of the preform. As a result, this invention enables the soldering of leaded components with a high-temperature lead-free solder alloy, yet at a soldering temperature well below the melting temperature of the alloy. The lower-temperature alloy of the preform can be selected so that the soldering temperature is not higher than the reflow temperatures of tin-lead alloys with which all of the electronic components on the same circuit board were designed and validated. Consequently, the invention avoids the concern that the temperature required to reflow a lead-free solder alloy for a leaded component could potentially damage the component being soldered as well as surrounding components on the same circuit board. The invention also avoids the necessity for revalidating components and processes at higher temperatures.
Other objects and advantages of this invention will be better appreciated from the following detailed description.